Reconstituted collagen is widely used in the manufacture of tubular sausage casings. The collagen used in these casings is usually derived from the corium layer of bovine hides. The bovine hides are processed to remove the collagen fibers or fibrils from the hides, usually by an acid-swelling process. The acid gel made by this process is then extruded into a casing and neutralized by the injection of gaseous ammonia or by contact with a liquid salt solution. The casing is washed in water to remove the neutralization salts and then treated with a crosslinking agent and plasticized. Examples of such processes are disclosed in U.S. Pat. Nos. 3,535,125 and 3,821,439. Although such casings have been commercially successful, they suffer certain disadvantages. These disadvantages relate to the inherent chemical nature of the collagen. As collagen is a hydrophilic polymer of amino acids, it is capable of absorbing large amounts of free water and swelling. The extent of the ability of collagen to swell and to form gels is dependent on two factors in particular. These are the pH of the gel and the chemical treatment processes to which the collagen has been subjected prior to the formation of the gel. Processes such as liming eliminate certain hydrophilic residues in the collagen and alter the swelling properties of the native collagen. Casings prepared from limed hides or hides which have been dehaired with lime have a tendency to absorb water from the meat emulsions with which they are stuffed. The absorbed water hydrates the casing, the casing becomes opaque, thereby blocking the transmission of meat colors through the casing. Transparent casing would be more acceptable to the meat industry, as it would more clearly show the meat color of the meat emulsion. It is possible to improve the translucency of the casing by using chemical cross-linking agents to make the casing more water resistant. Such agents, however, have a tendency to make the casing tougher and, therefore, more difficult to chew.
Another characteristic of the collagen casing that could be improved is the tendency of the casing to weep. Weeping is the passage of a large amount of moisture from the meat emulsion through the casing where it has a tendency to bead on the surface of the casing. Weeping is undesirable, because it is not attractive to consumers in selecting sausage from retail stores, and because it also provides moisture which allows various microorganisms to grow on the surface of the casing which leads to spoilage of the sausage.
The following patents disclosed the use of collagen derived from pork skin. U.S. Pat. No. 4,196,223 discloses a process of preparing sausage casing from pigskin. The collagen is prepared from the pigskin by swelling the pigskins with dilute hydrochloric acid, grinding the pigskins and homogenizing the ground pigskin into a gel which is then extruded. The casing produced by this process does not have adequate strength to be used in commercial stuffing equipment.
Canadian Pat. No. 695,243 discloses the manufacture of a water-dispersable collagen from fresh pigskins to form a water-dispersable collagen which would be useful as a dip coating. The product of this patent is not a sausage casing.